The successful outcome of allogeneic hematopoietic stem-cell transplantation (HSCT) depends on the complete reconstitution of the host's immune system by donor-derived T cells. While an intact thymus is critically important for T cell reconstitution, thymus-dependent T cell repertoire restoration may be limited by changes in the thymic epithelium, brought about by transplant-mediated toxicities from chemotherapy/radiation regimens or graft vs. host disease (GVHD). The aim of this proposal is to examine whether boosting the thymic epithelium by in vivo administration of epithelial-cell-trophic-factors, exerts beneficial effects on T cell regeneration and prevention of GVHD following allogeneic bone marrow transplantation (allo-BMT). The trophic factors we will use in these experiments include keratinocyte growth factor (KGF) and receptor activator of NF?B ligand (RANKL) in conjunction to a genetically modified animal model deficient in expression of the signaling protein Sin. KGF is a potent epithelial cell mitogen which has been shown to expand the cortical and medullary thymic epithelium, increase thymopoiesis, and prevent the development of GVHD in preclinical mouse models. RANKL on the other hand, is important for development and differentiation of a subpopulation of medullary thymic epithelial cells (mTECs) involved in suppressing autoimmunity through elimination of autoreactive T cell clones. We recently found that Sin is expressed in the thymic epithelium and Sin deficiency inhibits KGF- and RANKL-induced expansion of thymic epithelial cells. In this proposal we will test the following hypothesis: KGF- and RANKL-mediated expansion of the thymic epithelium leads to better T cell reconstitution and protection from GVHD following allo-BMT. Sin deficiency inhibits KGF/RANKL signaling and ameliorates the beneficial effects of these factors after transplantation. To test this hypothesis we will: 1) Use allo-BMT mouse models in the Sin-deficient background to examine whether Sin is required for KGF-mediated a) T cell repertoire regeneration; b) reconstitution of T cell-mediated immune responses and c) protection from GVHD. 2) Examine whether RANKL administration can also protect from allo-BMT-related toxicity, whether it acts independently of or in conjunction with KGF and if Sin is required for the effects of RANKL in allo-BMT. 3) Use Sin deficient mice to identify the thymic epithelial cell targets of KGF and RANKL as well as elucidate the molecular mechanisms through which Sin regulates KGF/RANKL-mediated thymic epithelial cell expansion. We anticipate that these experiments will provide novel insight into how the thymic epithelium regulates thymopoiesis under normal or transplantation conditions, may identify RANKL and Sin as regulators of thymic epithelia-mediated T cell reconstitution following allo-BMT, and may lead to the identification of novel strategies/targets to better manage the outcome of HSCT.